Level measurement devices for measuring the level of a liquid substance in a container using radar technique are used where special conditions make the use of other level measurement devices more or less insufficient. Such conditions may be the temperature in the container, the pressure in the container and the properties of the liquid. The liquid may, for example, be some kind of petroleum product or other chemical compound that has viscous properties that adversely may affect level measurement devices which are in contact with the liquid.
Therefore radar is suitable for measuring the level of the liquid, where a signal is transmitted from an antenna, reflected on the surface of the liquid, and received by the same or another antenna. Signal processing then provides a measure of the level. An example of such a device is disclosed in U.S. Pat. No. 4,665,403.
Such an antenna for sending and/or receiving the signal generally comprises a horn antenna having an antenna feeder which feeder in turn comprises a waveguide which at least partly is filled with a dielectric, such as PTFE (Polythetrafluorethylene). The transition from the waveguide to the horn is electrically matched in order to minimize the reflected power by means of a tapered part of the dielectric filler that protrudes in the horn. The waveguide is excited by means of a feeder pin or a pair of feeder pins, where the feeder pins may either protrude from a co-axial cable, or be microstrip lines protruding from a microstrip circuit board. The feeder pins may be excited to provide one linear polarization, two orthogonal linear polarizations or circular polarization.
The parts described above form a module, in the case of a microstrip circuit board, the electronic parts necessary for signal generation are provided on that circuit board, at the module. The module is mounted at the container in question by means of a flange that seals the container, where the flange encloses at least a part of the waveguide. Only the horn, the tapered part of the filler material and a part of the waveguide are normally exposed to the conditions that are present inside the container, as the filler material is provided with a sealing element that has a sealing function between the filler material and the waveguide. The horn antenna is preferably circular, having a circular waveguide mounted to it. The waveguide may then be circular all the way, or, alternatively, starting as a rectangular waveguide that is transformed to a circular waveguide. For a circular waveguide, the sealing between the filler material and the waveguide may be in the form of one ore more O-rings. This sealing thus prevents leakage of the containments of the container to the surroundings, at the same time as it affects the microwave signal in the waveguide to a very small extent. The waveguide is normally designed in such a way that the propagation mode following the fundamental mode has a cut-off frequency that is slightly above or, in some cases, above the frequency band used for the level measurements.
The total frequency span containing all the frequency bands normally used is approximately 6–26 GHz. For different reasons, for example that different materials are encased in the container, the frequency band that is used for the level measurements may vary. Then the size of the horn, the diameter of the waveguide and the shape of the tapered part of the filler material varies accordingly. The tapered part is normally either shaped as a cone with a more or less sharp tip for lower frequencies or as a convexly protruding lens for higher frequencies. Other shapes may also occur.
Having to use different frequency bands means that different models of flanges and sealing elements have to be provided, leading to higher costs for stock-keeping of these different models. This means that the product becomes more expensive for the costumer, who has to choose one of the models available, and who will have to choose another model should the need for change of frequency band occur.